# Calculating Current Through JFET Transistor System

• Hybird
In summary, the conversation discusses a problem of calculating the current through a system consisting of a JFET transistor with the source grounded, a reverse biased gate with a 3v potential, and a 2000 ohm resistor in series with a positive 20v potential. The desired current through the load resistor is 2mA, which would lead to a resistance of 8000 ohms. However, the speaker is unsure how to determine this without more information about the transistor's characteristics.

#### Hybird

I have a problem of calculating the current through a system. It is a simple JFET transistor with the source grounded. The gate is reverse biased with a 3v potential, and the drain has a 2000 ohm resistor in series then a positive 20v potential in series which is of course grounded. How would I go about finding the current through the load resistor?

...Assuming an n-type Si transistor

Also I don't have any other information.. I know that the more and more you apply a negative voltage on the gate, the smaller the channel gets which leads to more resistance across the source-drain channel. The answer I am lookin for is 2mA.. that would lead to a resistance of 8000 ohms, cause we still have a 2000 ohm resistor in series therefore 10000 ohm and with a voltage across the source-drain channel of 20 volts.. you get your 2mA answer, I don't see how you can get this bridge voltage without knowing something about a family of curves or the like?

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To calculate the current through the load resistor in this JFET transistor system, we can use Ohm's Law and the current-voltage relationship of a JFET transistor.

First, we need to determine the drain-source voltage (Vds) of the JFET transistor. Since the gate is reverse biased with a 3v potential and the source is grounded, the Vds can be calculated as 20V - 3V = 17V.

Next, we can use the current-voltage relationship of a JFET transistor, which states that the drain current (Id) is equal to the transconductance (gm) multiplied by the square of the gate-source voltage (Vgs) minus the threshold voltage (Vt). In this case, we can assume that the threshold voltage is zero since the transistor is in saturation mode.

Therefore, Id = gm * (Vgs)^2

To find the transconductance, we can use the formula gm = 2 * √(Idss * (Vgs - Vt)), where Idss is the maximum drain current when Vgs = 0.

Since the source is grounded, Vgs = 3V. Assuming Idss = 10mA, we can calculate gm as 2 * √(10mA * (3V - 0V)) = 6mA/V.

Now, we can plug in our values to calculate the drain current (Id): Id = 6mA/V * (3V)^2 = 54mA.

Finally, we can use Ohm's Law to find the current through the load resistor (I = V/R). Since the load resistor has a value of 2000 ohms and the drain-source voltage is 17V, the current through the load resistor is 17V/2000 ohms = 8.5mA.

Therefore, the current through the load resistor in this JFET transistor system is 8.5mA.

## What is a JFET transistor system?

A JFET (Junction Field Effect Transistor) system is a type of transistor that uses a voltage-controlled electric field to regulate the flow of current. It is commonly used in electronic circuits to amplify or switch signals.

## How is the current through a JFET transistor system calculated?

The current through a JFET transistor system can be calculated using Ohm's Law, which states that current (I) is equal to voltage (V) divided by resistance (R). In this case, the voltage is the voltage applied to the gate of the JFET and the resistance is the resistance of the channel between the source and drain terminals.

## What factors affect the current through a JFET transistor system?

The current through a JFET transistor system is affected by the voltage applied to the gate, the resistance of the channel, and the type and characteristics of the JFET itself. Other external factors such as temperature and noise can also have an impact on the current.

## How does the current through a JFET transistor system change with varying input voltage?

The current through a JFET transistor system is inversely proportional to the voltage applied to the gate. As the input voltage increases, the resistance of the channel decreases, allowing more current to flow. Conversely, as the input voltage decreases, the resistance increases and the current decreases.

## Are there any limitations to calculating current through a JFET transistor system?

Yes, there are some limitations to calculating current through a JFET transistor system. These include variations in the manufacturing process of the JFET, temperature changes, and external noise. Additionally, the model used for calculating current may not accurately reflect the behavior of the specific JFET being used.

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